This invention relates to an exhaust purifier for purifying hydrocarbons (HC) in the exhaust gas of an engine using an HC adsorbing unit and a three-way catalyst.
With regard to an exhaust purifier of a vehicle engine, Tokkai Hei 7-332073 published by the Japanese Patent Office in 1995 discloses an exhaust purifier wherein a three-way catalyst is disposed at a predetermined distance downstream of an HC adsorbing unit which adsorbs hydrocarbons (HC) in low temperature exhaust. According to this prior art, HC in the exhaust is adsorbed by the HC adsorbing unit when the engine is cold, and the activated three-way catalyst oxidizes HC desorbed from the HC adsorbing unit when engine warm-up is complete.
The reason why a space is left between the HC adsorbing unit and three-way catalyst is to avoid these catalysts reacting together under high temperature conditions.
This prior art further discloses that three-way catalysts having different compositions are installed downstream and upstream of the HC adsorbing unit so as to be able to oxidize all hydrocarbons, i.e., olefin hydrocarbons, paraffin group hydrocarbons and aromatic hydrocarbons.
HC adsorbing units require a predetermined capacity in order to provide the necessary adsorption performance. If the capacity of the HC adsorbing unit is increased, the heat capacity, i.e., the heat mass, also becomes large, and as a result, the exhaust gas temperature at the inlet of the three-way catalyst downstream of the HC adsorbing unit falls, and activation of the three-way catalyst is slower.
However, if activation of the three-way catalyst is not completed at the stage when desorption of HC from the HC adsorbing unit has begun, desorbed HC will be discharged into the atmosphere without being purified.
It is therefore an object of this invention to enhance the HC oxidation capability of a three-way catalyst while maintaining the adsorption capability of an HC adsorbing unit, in an exhaust purifier in which the HC adsorbing unit and a three-way catalyst are disposed in series.
It is a further object of this invention to facilitate manufacture of an exhaust purifier in which plural catalysts or adsorbing units are disposed in series.
In order to achieve the above objects, this invention provides an exhaust purifier disposed in an exhaust passage of an engine, comprising a shell having an inlet and an outlet connected to the exhaust passage, a plurality of hydrocarbon adsorbing units accommodated in the shell, and a plurality of three-way catalysts accommodated in the shell. The adsorbing units and catalysts are alternately disposed in series and one of the adsorbing units is disposed at the nearest position to the inlet.
This invention also provides a manufacturing method of the exhaust purifier interposed in an exhaust passage of an engine. The method comprises fixing plural HC adsorbing units at a predetermined distance apart to a cylindrical first element, fixing plural three-way catalysts at a predetermined distance apart to a cylindrical second element, and fixing the first element and second element to each other such that the HC adsorbing units and the three-way catalysts are alternately accommodated in a substantially cylindrical shell formed by the first element and the second element.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.